1. Field of Application
The present invention relates to a vehicle-mounted electrical generating system having two voltage systems that operate at respectively different voltages.
2. Description of Prior Art
In recent years, as requirements have arisen for vehicle electrical generating systems capable of to supplying power to an electrical load at a high voltage, in addition to supplying power at the low voltage of a conventional vehicle electrical generating system, types of vehicle-mounted electrical generating system have been proposed which incorporate multiple-voltage power sources. An example of such a prior art proposal is in Japanese patent publication No. 2001-333507 (pages 4 to 10, FIGS. 1 to 18). In that patent, a vehicle-mounted electrical generating system is proposed having two electric generators; a first generator which supplies power to a load that is rated at approximately 12 V, and a second generator which supplies an output voltage that varies over a wide range, from high to low values. The function of the second electric generator is to supply power to a DC motor that drives the rear wheels of the vehicle, with that generator producing a DC supply voltage which can vary within a range extending up to 50 V, for example. The respective outputs of the first and second electric generators are in effect connected in parallel (although mutually isolated by diodes) for supplying the field current of the field winding of the second electric generator (more specifically, the field winding of the alternator of the second electric generator). That is, the field current is supplied from the first electric generator or the battery (at the approximately 12 V battery voltage) when the output voltage of the second electric generator is lower than the battery voltage, and is supplied from the output of the second electric generator itself when that output voltage is higher than the battery voltage.
With an electric generator, when the density of magnetic flux in the magnetic circuit of the generator is low, any increase in the amount of magnetic excitation (i.e., resulting from an increase in the field current) will result in an increase in the magnetic flux density. However when the magnetic flux density reaches a certain level, no increase in the flux density will be produced by increasing the degree of magnetic excitation. That is to say, at a certain value of ampere-turns of magnetic excitation force, the output electrical power of the generator will not increase beyond a specific saturation value as the field current is increased, due to magnetic saturation in the magnetic circuits of the windings of the generator.
For that reason, it is preferred practice to design an electric generator to have a field winding with a value of resistance and number of turns such that saturation of the magnetic flux density occurs when a specific maximum field current is passed, and to operate the electric generator as far as possible such that the field current is maintained substantially close to that maximum value at which saturation begins to occur.
However in the case of the second electric generator of the above prior art example, the range of variation of the voltage applied to the field winding (i.e., when that is the output voltage of the second electric generator itself) is large, so that it is not possible to design the field winding such that the aforementioned condition that the field current be close to the saturation value will be maintained under various different operating conditions.
Furthermore, in order to reduce the amount of heat that is generated in the field winding of the second electric generator and in a switching element (in general, an FET) that controls the field current of that electric generator, it is necessary to establish a high value of resistance for the field winding of the second generator, in order to limit the maximum current that will flow through the field winding and the switching element when a maximum value of output voltage is being produced by the second electric generator.
Hence, it is not possible to utilize a field winding having a low value of resistance, in order to achieve a high value of magnetic excitation. In addition, the inductance of the field winding becomes large, so that delays occur in effecting changing of the level of magnetic excitation of the field winding by on/off switching control of the field current. Moreover, due to the high inductance of the field winding, high levels of electrical noise and large-amplitude voltage surges are produced by the second electric generator.
In particular, when the second electric generator is driving a large electrical load, high-amplitude voltage spikes will appear in the output voltage of that electric generator each time that the load is disconnected from the output terminals of the generator. These voltage spikes can damage a regulator (i.e., voltage control apparatus) that controls the switching element of the field winding of the second electric generator, or damage the rectifier circuit of that generator.
Furthermore with the second electric generator of the above prior art example, due to the fact that a high voltage is applied to the field winding, components such as the brushes and slip rings which transfer the field current to the field winding and which are exposed to the atmosphere, are subject to corrosion, due to intrusion of dust particles, etc., caused by the high voltage. Hence, such an electric generator has a low resistance to the adverse effects of the generator environment. This is especially true when the electric generator is a fan-cooled type, in which a flow of cooling air is passed through the interior of the generator.